Method for polishing a substrate surface

ABSTRACT

According to one aspect of the invention, an improved process for preparing a surface of substrate is provided wherein the surface of the substrate is prepared for a chemical mechanical polishing (CMP) process, the CMP process is performed on the surface of the substrate, and the surface of the substrate is finished to clear the substrate surface of any active ingredients from the CMP process. Also, an improved substrate produced by the method is provided. According to one aspect of the invention, particular polishing materials and procedures may be used that allow for increased quality of AlN substrate surfaces.

RELATED APPLICATIONS

This application is a continuation (CON) of U.S. application Ser. No.10/300,481 entitled “METHOD FOR POLISHING A SUBSTRATE SURFACE,” filed onNov. 20, 2002 now U.S. Pat. No. 7,037,838, which claims priority under35 U.S.C. § 119(e) to U.S. Provisional Application Ser. No. 60/331,868,entitled “CHEMICAL MECHANICAL POLISHING (CMP) PROCESS,” filed on Nov.20, 2001, both applications are herein incorporated by reference intheir entirety.

FEDERALLY SPONSORED RESEARCH

This invention was made in part with government support under grantnumber F33615-98-C-1325 from the United States Air Force, Air ForceResearch Laboratory, and grant numbers N00014-00-M-0160 andN00014-01-C-0232 from the Department of the Navy, Office of NavalResearch. The government may have certain rights in this invention.

FIELD OF THE INVENTION

The present invention relates generally to the preparation ofsemiconductor substrate surfaces, and more specifically to preparingsubstrate surfaces using chemical mechanical polishing (CMP).

BACKGROUND OF THE INVENTION

Chemical mechanical polishing (CMP) is a process that is used to polishsurfaces of semiconductor wafers. In particular, CMP employs bothphysical and chemical forces to polish wafer surfaces. First, a loadforce is applied to the back of a wafer while it rests on a polishingpad. Both the pad and the wafer are then counter-rotated while slurrycontaining both abrasives and reactive chemicals is passed underneath.

CMP is typically used to planarize a surface of a wafer for the purposeof creating an integrated circuit. CMP has emerged as the planarizationmethod of choice because of its ability to planarize over longer lengthsthan traditional planarization techniques. As discussed, CMP is acombination of a chemical reaction and mechanical action. First, theslurry weakens the wafer surface and the slurry particles, in additionto particles located on the pad, complete material removal from thewafer surface.

It is desired to use aluminum nitride (AlN) as a substrate material forcreating commercial semiconductor devices, however, there are problemsin processing AlN surfaces to derive substrate surfaces suitable forepitaxial growth. For example, current CMP processes damage the AlNmaterial making it unsuitable for epitaxial growth. It would bebeneficial to have an improved method for polishing AlN wafers thatproduces a quality surface suitable for epitaxial growth.

SUMMARY OF THE INVENTION

One illustrative embodiment of the invention is directed to a method ofpreparing a surface of a substrate, the method comprising the acts ofpreparing the surface of the substrate for a chemical mechanicalpolishing (CMP) process, performing the CMP process on the surface ofthe substrate, and finishing the surface of the substrate, wherein theact of performing the CMP process further comprises an act of applyingan abrasive suspension in a solution during the CMP process. Accordingto one aspect of the invention, a method is provided wherein thesubstrate is aluminum nitride (AlN).

According to another aspect of the invention, the abrasive suspension inthe solution further comprises a silica suspension in a hydroxide(basic) solution. According to another aspect of the invention, theabrasive suspension includes a KOH-based slurry. According to one aspectof the invention, the abrasive suspension in the solution furthercomprises a silica particles suspended in an ammonia-based slurry.

According to another aspect of the invention, the act of performing theCMP process includes an act of applying the abrasive suspension at arate of approximately 0.5 mL per minute for a circular 8.0″ diameterpolishing surface. According to one aspect of the invention, the act ofperforming the CMP process includes an act of maintaining a polishingspeed of the sample in a range of approximately 13 to 18 m/sec.

According to another aspect of the invention, the act of preparingincludes cleaning a polishing apparatus prior to polishing the surfaceof the substrate to substantially reduce contamination of the polishingsurface. According to one aspect of the invention, the act of finishingthe surface of the substrate includes an act of substantially rinsingthe abrasive suspension from the surface of the substrate. According toanother aspect of the invention, the method further comprises cleaning,during the preparing acts, the surface of the substrate with a solvent.According to another aspect of the invention, the substrate is anon-axis, Al-polarity, c-face surface, and the abrasive suspension has apH value of approximately 10.5 or greater. According to another aspectof the invention, an substrate is provided produced by the method.According to another aspect of the invention, a device having asubstrate produced by the method is provided.

According to another aspect of the invention, a method is provided forpreparing a surface of a substrate. The method comprises acts ofpreparing the surface of the substrate for a chemical mechanicalpolishing (CMP) process, the act of preparing comprising acts ofdetermining an orientation of the substrate; and performing a removal ofthe surface of the substrate based on the orientation. According toanother aspect of the invention, the substrate is AlN, and the act ofdetermining an orientation includes an act of determining, for the AlNsubstrate, at least one face of the AlN substrate and its orientation tothe surface to be polished. According to another aspect, the act ofperforming a removal includes an act of determining a period of removalbased on the orientation.

According to another aspect of the invention, the act of performing aremoval includes an act of removing exposed off-axis material from thesubstrate surface. According to another aspect, the act of removing theexposed off-axis material includes removing between 50 and 100 μm ofmaterial from the substrate surface.

According to another aspect, the substrate includes a surface normal tobe polished, and the act of determining an orientation includesdetermining, for a given face of the substrate, an orientation of thenormal surface with respect to the given face. According to anotheraspect of the invention, the act of performing a removal of the surfaceof the substrate based on the orientation of the normal surface withrespect to the given face.

According to another aspect, the substrate includes an Al-polarity sideof c-face substrate, and the act of performing a removal includesperforming at least one of a dry grinding and polishing using a polymerdiamond suspension to prepare the Al-polarity side of the c-facesubstrate. According to another aspect of the invention, the AlNsubstrate includes a non-polar surface, and the act or performing aremoval further comprises an act of removing between 10 and 20 μm of thenon-polar surface.

According to another aspect of the invention, the method furthercomprises an act of performing the chemical mechanical polishing (CMP)process, and a polishing of the substrate surface is performed based onthe orientation. According to another aspect, the polishing furthercomprises an act of polishing the substrate surface with a slurry havinga pH value, and the slurry being selected based on the face of thesubstrate and the pH value.

According to another aspect, the substrate is AlN, and the substratesurface is the on-axis Al-polarity, c-face surface, and the pH value ofthe selected slurry is greater than 10.5. According to another aspect ofthe invention, an substrate is provided produced by the method.According to another aspect of the invention, a device having asubstrate produced by the method is provided.

Further features and advantages of the present invention as well as thestructure and operation of various embodiments of the present inventionare described in detail below with reference to the accompanyingdrawings. In the drawings, like reference numerals indicate like orfunctionally similar elements. Additionally, the left-most one or twodigits of a reference numeral identifies the drawing in which thereference numeral first appears.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is pointed out with particularity in the appended claims.The above and further advantages of this invention may be betterunderstood by referring to the following description when taken inconjunction with the accompanying drawings in which similar referencenumbers indicate the same or similar elements.

In the drawings,

FIG. 1 shows a conventional CMP apparatus that may be used to polishsubstrate surfaces in a CMP process according to one embodiment of theinvention;

FIG. 2 shows a flow chart of a process used to polish a substratesurface according to one embodiment of the invention;

FIG. 3 shows a more detailed process for polishing a substrate surfaceaccording to one embodiment of the invention;

FIG. 4 shows an AFM image of an Al-polarity c-face of an AlN substrateafter performing CMP polishing according to one embodiment of theinvention; and

FIG. 5 shows an AFM image of an AlN substrate surface tilted 45 degreeswith respect to a c-axis direction of the AlN substrate, the surfacepictured after performing CMP polishing according to one embodiment ofthe invention.

DETAILED DESCRIPTION

The use of single crystal III-nitrides allow improved epitaxial growth,improved thermal and chemical compatibility, as well as improved thermalconductivity. Applications of wide-bandgap and high-temperaturesemiconductors including the development of blue/UV solid-state chargeinjection lasers, UV optical sources and detectors, high power microwavedevices, high power switches, and high temperature applications.However, there is a need to obtain a method for suitably preparingsurfaces of aluminum nitride (AlN) substrates for epitaxial growth toproduce these devices. For example, nitrides (e.g. AlN, Al_(x),Ga_((1-x))N, etc.) may be epitaxially grown on single crystal AlNsubstrates by, for example, Organometallic Vapor Phase Epitaxy (OMVPE)and other formation processes.

Device fabrication on AlN substrates generally involves the epitaxialgrowth of a device layer. The quality of this device layer depends onthe quality of the surface of the AlN substrate. Despite the bestpolishing techniques, the wafer surface can retain polishing defectssuch as micro-damage and surface roughness. Surface defects are known toeffect the quality of finished semiconductors prepared through epitaxialgrowth. It is believed that conventional polishing procedures damage theAlN surface both at a submicron scale and just under the AlN surface insuch a way that the substrate is unsuitable for epitaxial growth.Difficulty arises in preparing such surfaces because the AlN surface isunable to be polished using conventional CMP process without introducingsurface defects. Because conventional CMP techniques damage the AlNsurface, a method that produces an atomically smooth surface suitablefor epitaxial growth for AlN crystals is desired.

According to one aspect of the invention, an improved process forpreparing a surface of a substrate is provided wherein the surface ofthe substrate is prepared for a chemical mechanical polishing (CMP)process, the CMP process is performed on the surface of the substrate,and the surface of the substrate is finished to clear the substratesurface of any active ingredients from the CMP process. Also, animproved substrate produced by the method is provided. According to oneaspect of the invention, particular polishing materials and proceduresmay be used that allow for increased quality of AlN substrate surfaces.

To employ AlN crystals as substrates for epitaxial growth of othercompound semiconductors such as GaN, InN, or 2H—SiC, large area, highperfection AlN wafers are needed. After growing AlN boules, the boulesare generally cut into wafers using annular diamond saws or wire sawsand then the surface of each is ground and polished flat. According toone embodiment of the invention, AlN crystal surfaces (e.g., the a, +cor −c surfaces of AlN) are polished using CMP to derive surfacessuitable for epitaxial growth. Optimally, these polished surfaces areflat, highly perfect surfaces with a minimum of scratches, pits, ordamage dislocations and other subsurface damage. Also, these polishedsurfaces are also free of aluminum oxide coating or islands.

In any grinding process, the powder employed for grinding should have amicrohardness greater than or equal to that of the crystal being ground.The basic goal is to remove material from the crystal in a short periodof time. The polishing is intended to give a mirror-like finish to thesurface, and leave a damage-free crystal underneath. This polishingprocess is much like “chemical-mechanical-planarization” processes asare known in the art which are used most frequently to planarize wafersurfaces of multilayer devices. In this process, the polishing lubricantactually reacts slowly with the crystal being polished. The slurry andabrasive pad are designed to continuously remove the reaction productswithout appreciably damaging the underlying crystal.

In CMP, high elevation features of a wafer are selectively removed(i.e., material from high elevation features is removed more rapidlythan material at lower elevations), resulting in a surface having animproved planarity. Mechanical polishing, assisted by chemical action,produces such selective material removal. The process is performed bymounting the wafer face down on a carrier. The carrier is then pressedagainst the rotating platen containing a polishing pad. The carrieritself is also rotated. An abrasive-containing aqueous slurry is drippedonto the table, saturating the pad. Conventionally, the slurry comprisesabrasives of silica, alumina, ceria, or other nanometer-size particlessuspended in an alkaline or acidic medium.

According to one aspect of the invention, it is realized thatorientation affects mechanical preparation of a substrate surface priorto CMP processing. So-called c-face substrates are produced when the AlNcrystal is sliced perpendicular to the c-axis of the crystal. Thesec-face substrates are polar and the surfaces on the two sides of thesubstrate will have quite different properties. One side of thesubstrate is aluminum (Al) terminated (or so-called Al-polarity, c-face)and the other side is nitrogen (N) terminated (or so-called N-polarity,c-face).

When the AlN crystal is cut so that the c-axis of the crystal iscontained in the plane of the substrate, a non-polar substrate isproduced wherein the two surfaces have identical behavior. Substratesthat are cut at some angle away from the c-axis (other than 90°) alsodemonstrate some polarity effects, but these effects are not as strongas the effects of c-face substrates. C-face substrates are currentlybeing used to produce electronic devices that take advantage of polareffects to create a high-density electron gas without any or withminimal doping. However, optoelectronic devices are expected to bebetter if non-polar substrates are used.

According to one embodiment of the invention, it is realized thatsubstantial differences exist for optimal preparation of the substratessurfaces with different crystallographic orientations. In the case of anAlN substrate, it is realized that the Al-terminated, c-face is notreactive with water, but the N-terminated c-face is reactive with water,along with non-polar faces. During wet lapping and polishing, theAl-polarity face tends to chip under the same conditions that arewell-suited to mechanically polish the non Al-polarity faces and forAl-polarity faces where the c-axis is oriented 20 degrees or more awayfrom the surface normal of the substrate. (Effectively, wet mechanicalprocessing used on orientations other than the on-axis c-face isactually a chemical mechanical processing with a fixed abrasive).According to one embodiment of the invention, dry grinding and polymerdiamond suspensions are used to prepare the Al-polarity side of c-facesubstrates that are close to on-axis (misorientation less thanapproximately 20°). Even under strictly mechanical polishing, theremoval rates observed for the on-axis, Al-polarity surface is lowerthan for all other crystal orientations.

According to one embodiment of the invention, it is realized that theamount of material that should be removed during the mechanicalprocessing prior to fine mechanical polishing and CMP depends uponquality of the saw cut. Typically between 50 and 100 μm must be removedto level the irregular cut of the saw for an annular saw. At this point,the sample is generally planar and should have a low pit density, andthe sample may be subjected to fine mechanical processing. In this step,it is realized, according to one embodiment, that planarity of thesamples be brought to the highest level for c-face, Al-polaritysubstrate surfaces, because it is realized that any exposed off-axismaterial will lead to undercutting of the desired surface during CMP.For non-polar surfaces, the high chemical reactivity allowsplanarization during CMP (few microscopically viewable pits andscratches are allowable and does not cause the CMP undercutting effectson the Al-polarity surfaces. Fine mechanical processing generallyinvolves a measured removal totally between 10 and 20 μm of samplethickness.

According to one embodiment of the invention, it is realized that theremoval rate during CMP is very much a function of the crystallographicorientation of the substrate surface. For the Al-polarity, c-facesubstrates, the removal rate increases from around 1 μm per hour to over10 μm per hour as the angle between the surface normal and the c-axis isincreased from near zero to over 20°. For the non-polar and N-polarity,c-face surfaces, the removal rate is mechanically limited by theabrasive particles in the slurry.

EXAMPLE

FIG. 1 shows a conventional CMP apparatus that may be used to polishsubstrate surfaces in a CMP process according to one embodiment of theinvention. A wafer 101 to be polished is mounted on a wafer carrier 102which generally includes a backing film 109 positioned between wafer 101and a chuck 108 which holds the wafer and the wafer carrier 102. Wafercarrier 102 is rotated by a spindle 105. A force is applied to the wafercarrier 102 to contact the wafer 101 with one or more polishing pads103A-103B. The one or more polishing pads 103A-103B are adhered to aplaten 106 which also rotates. Further, a slurry 107 is applied to thepad 103A-103B and the drip rate is controlled, for example, by a controlflow dispenser (not shown).

It should be appreciated that other CMP tools and/or polishing apparatusmay be used and the invention is not limited to any particular CMP toolsor polishing configurations.

According to one aspect of the invention, the CMP process may involvepolishing the substrate using a slurry, the slurry comprising anabrasive suspension in a solution such that the slurry is capable ofetching the substrate surface and creating a finished surface suitablefor epitaxial growth. For example, a silica suspension in a solution maybe used. This solution may be, for example, a hydroxide (basic)solution. Such a slurry is available commercially, for example, as theKOH-based CMP slurry known in the art as SS-25 (Semi-Sperse 25),available from Cabot Microelectronics or the Syton slurry available fromMonsanto.

According to one embodiment of the invention, it is realized that ahigher pH KOH slurry works better for the Al-polarity, c-face surface.The SS25 slurry has a pH of 11.0. Its high pH distinguishes it fromother commercially availably KOH slurries (like Syton and Glanzox) whichhave lower pH. For the on-axis Al-polarity, c-face surface, it isrealized, according to one embodiment of the invention, that the pHshould be over 10.5 to obtain observable removal rates.

It should be appreciated that other slurry types may be used. Forexample, diamond, silicon carbide, or other material may be used in aslurry. Also, other commercially-available slurries are available (e.g.,SS-25-E and SS-225 (ammonia hydroxid-based) both of which are availablefrom Cabot Microelectronics, AM 100 and Rodel 2371 (ammonia-basedslurries)).

Such chemical/mechanical polishing methods are particularly suitable forpreparing very hard surfaces, such as aluminum nitride (AlN) surfaces.It should be appreciated that various aspects of the invention reduce anamount of impurities and surface defects introduced into the AlN crystalsubstrate by the polishing procedure.

FIG. 2 shows a process 200 for polishing a substrate according to oneembodiment of the invention. At block 201, process 200 begins. At block202, the substrate is prepared for polishing. This may include removalof contamination from the surface of the substrate, the polishingapparatus and environment. Such contamination is generally responsiblefor the introduction of surface defects during the polishing process. Atblock 203, a CMP process is performed on the substrate surface. In thecase of AlN, the CMP process may include polishing the surface using aslurry having a hydroxide solution. For example, the SS-25 slurryavailable from Cabot Microelectronics may be used. It should beappreciated that the invention is not limited to the type of slurry, andthat other appropriate slurries may be used as discussed above. At block204, the polished sample is finished. This may involve, for example,removal of the sample from the polishing apparatus and carefullycleaning the sample to remove any reactive agents introduced by thepolishing process. At block 205, process 200 ends.

FIG. 3 shows a more detailed process 300 for polishing a substrateaccording to one embodiment of the invention. At block 301, process 300begins.

According to one embodiment of the invention, the AlN substrate isprepared for a CMP process. More particularly, the environment and waferare cleaned of possible contaminants, and the polishing apparatusadjusted to apply the correct polishing force to the wafer. Thefollowing example is a procedure for preparing the substrate accordingto one embodiment of the invention:

I. Preparation of Substrate and Apparatus for CMP Process

1. The sample surface and mounting block should be cleaned ofsubstantially all possible contamination particles (e.g., wax, dirty,and larger mechanical grit particles). This is typically done with, forexample, organic solvent (e.g., by acetone and/or methanol) cleaningperformed at block 302. Also, any remaining particles from previousprocessings (e.g., CMP fluid residue) should be cleaned from allsurfaces that contact the polishing area so as to reduce the likelihoodof contaminating the polishing surface and potentially damaging thesubstrate surface.

2. The mounting block is then prepared to hold the sample at block 303.For example, the sample may be adhered to a surface of the mountingblock. The mounting block is then switched from the “mechanical” processfixture to the “CMP” process fixture. According to one embodiment of theinvention, these fixtures may be identical items; their use may besegregated to avoid cross contamination between the mechanical andchemical mechanical polishing.

3. The mounted sample may have the vertical offset corrected to allowthe sample appropriate contact with the polishing surface. A mass toensure proper down force can be added to the block at 304. Pressure in arange of 5×10⁵ to 7×10⁵ N/m² have been shown to be suitable. These areconditions that have yielded the most consistent results for bothnon-polar and polar faces. However, it should be appreciated that theinvention is not limited to any particular set of conditions.

According to one embodiment of the invention, a CMP process may beperformed on the prepared substrate surface. In one aspect of theinvention, SS-25 (Semi-Sperse 25) slurry available from the CabotMicroelectronics Corporation may be used to polish an AlN substrate.

II. CMP Process

The following is a detailed sequence of steps, according to oneembodiment of the invention, to prepare an AlN substrate surface and thedrip rates for a CMP process according to one embodiment of theinvention.

-   -   A new “Multitex 1000” fine polishing pad or any other suitable        fine polishing pad should be applied to the polishing table        (platen). Generally, such a polishing pad has a self-adhesive        backing to adhere it to the platen surface. At block 305, the        polishing pad is adhered to the platen surface. For best        results, the polishing pad should be applied to the polishing        deck in such a way to ensure that the pad is not placed with        space behind the pad causing it to be uneven. For example, the        polishing pad may be rolled onto the table from the lead edge to        the other side with a cleaned rod so that spaces behind the pad        can be avoided.    -   At block 306, the pad surface may be cleaned. For example, the        pad may be cleaned with distilled water, saturated with water        and the empty pad and platen surface rotated (e.g., 45-60 RPM or        13 to 18 m/sec in the sample path) on the pad for 10 to 15        minutes. During this cleaning process, the sample (with the        substrate to be polished) should be lifted out of contact with        the polishing surface. During this time, the surface of the pad        should be thoroughly rinsed with filtered distilled water. This        is performed to ensure that both the pad and the fixture have        been completely cleaned of all dust and cross-contaminated        particles.    -   At block 307, the slurry drip rate and rotation of the fixture        should be set for polishing. During this rotation cleaning of        the surface, the drip rate should be set using an appropriate        flow control dispenser. The rate that has yielded the best and        most consistent polishing results has been 3 drops of SS-25        slurry and distilled water per minute (approximately 0.5        mL/minute of each of the CMP fluid and distilled water for a        circular 8.0″ diameter polishing surface). It should be        appreciated, however, that the invention is not limited to any        particular drip rate, and that drip rate depends on the type of        slurry used, pressure applied to the sample, and rotation speed.        With this average drip rate set, the SS-25 slurry should be        allowed to drip onto the polishing surface allowing the lapping        fixture to evenly spread the solution and fill the sample area        with a consistent level of abrasive particles and reactive        solution prior to polishing the substrate surface. The polishing        pad may be preconditioned in this manner prior to polishing for        approximately 30-60 minutes or any other appropriate time to        allow for substantially even distribution of slurry and etchant        over the surface of the pad.    -   Following the proper rate confirmation and with the observation        of proper rotation of the fixture, then the sample may be        lowered on the surface to begin the CMP process. At block, 308,        the sample is polished.

Although a typical process time (polishing time) for this step isapproximately 1 hour, a process time in a range of 5 minutes to 100hours has been shown to adequately polish a surface. It should beunderstood that the invention is not limited to a particular processtime, but rather, the process time range is provided by way of example.

Although the material removal depends highly upon the crystal qualityand the orientation of the sample, common values range from 0.1 μm (Alface c-axis) to 60 μm (N face c-axis) for 1 hour processing.

According to one embodiment of the invention, the sample is cleaned toremove any contamination from the CMP process, and to remove traces ofreacting chemicals from the substrate surface to stop any residualreactions with the substrate.

III. Finishing

-   1. At block 309, the sample is rinsed. At the end of the CMP    process, the sample should be lifted from the polishing surface and    immediately rinsed, for example, with distilled water. This is    performed to ensure that the reaction between the AlN and any of the    active ingredients in the slurry be stopped, and to remove abrasives    that may dry to the prepared surface.-   2. Following this rinsing, the sample is carefully cleaned at block    310 with suitable organic solvents to remove the agent bonding the    substrate to the substrate holder. Mounting waxes soluble in these    solvents and other common sample holding techniques may be used to    hold the substrates, as well as other adhesive film and vacuum    chucking techniques. Each of these holding techniques may need    different substrate debonding procedures at block 311, as is    well-known in the art.-   3. Following the CMP processing, the sample surface should be    cleaned very well at block 312. For example, the sample surface may    be cleaned prior to removal of the sample from the mounting block,    and then the entire sample may be cleaned when removed.-   4. At block 311, process 300 ends.

FIG. 4 shows an AFM image of the Al-polarity of the c-face substrateafter polishing according to one embodiment of the invention. Thesubstrate surface is oriented with its normal approximately 5° off thedirection of the c-axis. It can be seen that the surface is nearlyatomically flat. Analysis of a wider area shows that all mechanicaldamage has been removed. Similarly smooth surfaces may be obtained forthe N-polarity of a c-face substrate.

FIG. 5 shows an AFM image of a substrate after polishing according toone embodiment of the invention. The surface is tilted 45° with respectto the c-axis direction. As can be observed, the substrate surface isnearly atomically flat. Evidence of mechanical damage has been removed.The crossed box on the center was eliminated from the image for thestatistical analysis.

Having described several embodiments of the invention in detail, variousmodifications and improvements will readily occur to those skilled inthe art. Such modifications and improvements are intended to be withinthe spirit and scope of the invention. Accordingly, the foregoingdescription is by way of example only, and is not intended as limiting.The invention is limited only as defined by the following claims and theequivalents thereto.

1. A method of preparing a surface of a single-crystal aluminum nitride(AlN) substrate, the method comprising the acts of: preparing thesurface of the single-crystal AlN substrate for a chemical mechanicalpolishing (CMP) process; performing the CMP process on the surface ofthe single-crystal AlN substrate; and finishing the surface of thesingle-crystal AlN substrate, wherein the act of performing the CMPprocess comprises applying an abrasive suspension in a solutionconsisting essentially of a hydroxide during the CMP process.
 2. Themethod according to claim 1, wherein the solution has a pH value greaterthan approximately 10.5.
 3. The method according to claim 1, wherein theabrasive suspension in the solution comprises silica.
 4. The methodaccording to claim 1, wherein the hydroxide comprises KOH.
 5. The methodaccording to claim 1, wherein the hydroxide comprises ammonia hydroxide.6. The method according to claim 1, wherein preparing includes cleaninga polishing apparatus prior to polishing the surface of thesingle-crystal AlN substrate.
 7. The method according to claim 1,wherein finishing the surface of the single-crystal AlN substrateincludes substantially rinsing the abrasive suspension from the surfaceof the single-crystal AlN substrate.
 8. The method according to claim 1,wherein preparing the surface of the single-crystal AlN substratecomprises cleaning the surface of the single-crystal AlN substrate witha solvent.
 9. The method according to claim 1, wherein the surface ofthe single-crystal AlN substrate is non-polar.
 10. The method accordingto claim 1, wherein the surface of the single-crystal AlN substrate isan Al-terminated c-face.
 11. The method according to claim 1, whereinthe surface of the single-crystal AlN substrate is an N-terminatedc-face.
 12. The method according to claim 1, wherein an angle between anormal of the surface of the single-crystal AlN substrate and a c-axisis less than 90°.
 13. The method according to claim 8, wherein thesolvent is organic.
 14. The method according to claim 13, wherein thesolvent comprises at least one of acetone or methanol.
 15. The methodaccording to claim 1, wherein the solution has a pH value greater thanapproximately 9.5.
 16. The method according to claim 1, wherein thesolution has a pH value greater than approximately 10.2.